September 22 heralds the Autumn Equinox, marking the official end of summer and the beginning of the Autumn season in the Northern Hemisphere.
Daytime will become shorter and nights will become longer as the Earth tilts further and further on its axis, causing the Northern Hemisphere to spend less time under sunlight. The effect is reversed in the Southern Hemisphere, where today is the first day of Spring.
Coincidentally, the Moon and Jupiter are positioned in conjunction in the Cosmos today, so they will appear very close together in the night sky tonight and tomorrow.